1. Field of the Invention
The invention relates to collapse-resistant frame systems, as may be incorporated in bridge and building structures located in geographic regions likely to experience seismic activity.
2. Background Art
The prior art has recognized the desirability of providing building and bridge structures with collapse-resistant frame systems with self-centering capabilities to thereby reduce the likelihood of a catastrophic failure of such structures and relatively large residual displacements in the event of large seismic excitation. Typically, collapse-resistant building or bridge structures include interconnected columns and beams that are formed of steel or steel-reinforced concrete, whose load-deformation characteristics, for example, as a beam in flexure are generally governed by the material properties of steel or steel reinforcement in a concrete matrix. Because steel/reinforcing steel may be characterized as exhibiting xe2x80x9celastic/plasticxe2x80x9d behavior (so-called because of the relatively-low elastic strain capacity of the material, coupled with the material""s significant, additional plastic strain capacity at constant load as the material begins to yield plastically), the columns and beams of such known frame systems may likewise be expected to exhibit xe2x80x9celastic/plasticxe2x80x9d behavior. Thus, once the elastic strain limit of such columns and beams are exceeded, the behavior of such columns and beams is essentially governed by the rotation of a concentrated region of plastic deformation (the formation of a xe2x80x9cplastic hingexe2x80x9d). Thus, as the columns and beams yield plastically in respective regions of maximum stresses (the plastic hinges), the deformation localizes at the plastic hinges and the load can only marginally be increased due to possible strain-hardening of steel in the plastic deformation regime (the column or beam can no longer resist increases in applied load and, hence, continues to plastically deform with applied load until ultimate failure).
The prior art has sought to utilize plastic hinge formation in the beams of such known structures as a mechanism for dissipating seismic energy. However, in a typical building or bridge structure formed of steel or steel-reinforced concrete frame members, a large rotational deformation in particular columns (usually first-floor columns) of the structure in response to seismic excitation is necessary in order to activate the energy-dissipating capacity of the beams through plastic hinge formation. Such large rotational deformation in the columns is achieved through the formation of plastic hinges in the base of the columns (the formation of a plastic hinge becomes apparent in the flexural load-deformation behavior as a constant or only marginally increasing load at increasing plastic deformation). Unfortunately, such plastification of the base of the structure""s columns in conjunction with the formation of plastic hinges in adjacent beams may cause a collapse of the structure, even when the structure has been subjected to relatively limited lateral displacement, as confirmed by the collapse of many xe2x80x9csoftxe2x80x9d first-story frame systems in recent major earthquakes.
Still further, because yielding (in both the columns and the beams) is concentrated at the plastic hinge, the material must be capable of undergoing a large local plastic strain in order to accommodate the targeted displacement) of each frame member, while regions of the frame member outside the plastic hinge essentially remain elastic. In steel-reinforced concrete members, this localization is caused by a combination of the brittle stress-strain behavior of concrete and the elastic/plastic behavior of reinforcing steel. Beyond yielding of reinforcement, this particular section serves as a weak link and deformation (i.e., cracking) cannot be further distributed. The deformation capacity of the frame member is then limited by the localized failure of this weakened section, such as fracture or buckling of steel reinforcement.
In order to achieve a better distribution of strain than available with reinforced-concrete frame members, the prior art has proposed use of frame members made of a fiber-reinforced cementitious material known as ECC (Engineered Cementitious Composite). ECC contains a small volume fraction of polymeric fibers, typically less than about 2% by volume, and shows pseudo-strain hardening behavior accompanied by the formation of multiple cracking. ECC does not break after cracking (like concrete), but can increase its strength while deforming up to 4% strain in tension. In a frame member of steel-reinforced ECC (as opposed to steel-reinforced concrete), the unique properties of ECC result in a distribution of cracking in an extended plastic hinge region and cause a decrease in peak strain demand on the reinforcement. Consequently, steel-reinforced ECC frame elements have a larger deformation capacity than steel-reinforced concrete frame members, and seismic-excitation-induced flexural deformation is more evenly distributed along the member. However, the elastic/plastic material characteristics of the steel reinforcement still govern the load-deformation behavior of steel-reinforced ECC frame members and, hence, the use in a frame system of steel-reinforced ECC columns nonetheless results in deleterious plastic hinge formation at the column base.
Under another prior art approach, a steel-column-and-beam frame system employs weakened beams in order to focus plastic hinge deformation in a select region of each beam, for example, away from the beam-column joints (welds). However, such weakened-beam frame systems nonetheless require plastic hinge formation in the lowermost columns in order to activate energy dissipation through plastic hinge formation in the select region of the beams.
Another known approach is to incorporate mechanical devices and complex response control systems into the structures, which are similar to sophisticated suspension systems of automobiles. Such mechanical devices and control systems are expensive and require major maintenance efforts.
Accordingly, it would be desirable to provide frame systems for building and bridge structures that have improved collapse-resistance and self-centering capabilities after undergoing large deformations relative to known steel and steel-reinforced concrete frame systems.
Under the invention, a frame system for use, for example, in an earthquake-resistant building or bridge structure, includes a plurality of interconnected beams and columns, wherein the beams are formed of a first material that enables the beams to exhibit an elastic/plastic behavior, and wherein at least the lowermost free-standing columns (those lacking external lateral support, such as the first-story columns of a multi-story building structure, rather than laterally-bolstered sub-ground-level columns) are formed of a second material that enables such columns to exhibit a xe2x80x9cquasi-elasticxe2x80x9d behavior over a range of flexural deformation sufficient to allow plastic hinge formation in the adjacent beams. For frame systems including a plurality of tiers of beams, such as a multi-story building structure, the columns supporting the upper tiers preferably likewise exhibit quasi-elastic behavior relative to the material of the beams of the lowermost tier. By xe2x80x9cquasi-elasticxe2x80x9d behavior, it is meant that the behavior is characterized by proportionally increasing resistance to increasing load, notwithstanding the fact that the frame member formed of such material may not return to its exact original shape upon removal of such loads. In this way, the columns remain quasi-elastic and maintain vertical stability and self-centering capabilities of the structure that incorporates a frame system according to the invention, while plastic deformation and energy dissipation are assigned to the formation of plastic hinges in the beams.
Thus, in accordance with a feature of the invention, in the event of a lateral displacement of the upper tiers of the frame system, the quasi-elastic load-deformation behavior of the column material results in initial plastic hinge formation at the beam ends, such that the beams rather than the columns dissipate the structure-deforming energy up to the point where the beam material at the plastic hinges fails, thereby substantially preventing plastic hinge formation in the columns, for example, at the column base. The prevention of plastic hinge formation in the columns provides a substantial improvement in terms of safety while significantly reducing residual deformation and repair needs, even after strong seismic events, by maintaining the load-carrying capacity of the columns and preventing damage to critical column members. Repair costs are also reduced, because plastic deformation is forced into the beams, which can typically be repaired or replaced without interfering with the vertical stability of the entire structure.
While the invention contemplates any suitable material for the columns that provide the intended quasi-elastic behavior, an example of a suitable material is a composite of fiber-reinforced plastic (FRP) and an engineered cementitious composite (ECC) that itself comprises a mixture of cementitious material, and hydrophilic and/or hydrophobic fibers. More specifically, the ECC""s cementitious material is any suitable cement material such as conventional cements, or mixtures of conventional cements. Examples of suitable hydrophilic fibers include PVA (polyvinyl alcohol) fibers, EVOH (ethyl vinyl alcohol) fibers, polyvinyl acetate fibers, ethylene vinyl acetate, hydrophilic acrylic fibers, and acrylamide fibers. Examples of suitable hydrophobic fibers include polyethylene (Spectra) and polypropylene fibers. By way of example only, the hydrophilic and/or hydrophobic fibers in the ECC preferably comprise about 0.5 to about 10 volume percent of the ECC, more preferably in an amount of about 1 to about 3 volume percent, and most preferably in an amount of about 1 to about 2 volume percent. Preferably, the FRP reinforces the ECC matrix.
Similarly, while the invention contemplates any suitable material for the beams that provide the intended elastic/plastic behavior, examples of suitable materials for the elastic/plastic beams includes steel, steel-reinforced concrete, steel reinforced FRC (fiber-reinforced concrete), and steel-reinforced ECC.